1. Field of the Invention
The present invention relates to air moving devices, and in particular, to blowers of the type which are used with high efficiency (e.g., 90% or higher efficiency) furnaces for drawing air from outside of a building into the furnace to support combustion and to expel combustion exhaust products outside of the building. More particularly, the present invention relates to a blower which provides more efficient air flow through the blower housing with decreased blower noise.
2. Description of the Related Art
In high efficiency furnaces, standard chimney air-draw effects are not sufficient to assure the required air flow through the furnace heat exchangers, and therefore, high efficiency furnaces utilize draft inducer blowers to provide sufficient air flow through the furnace. In particular, the blowers of high efficiency furnaces pull flue gases through the furnace heat exchangers and then push the flue gases out through exhaust piping to the exterior of the building. The length of the flue piping is limited by the static pressure induced on the flue gases by the draft inducer blower, and higher static pressures typically allow longer runs of flue piping. One measure of the efficiency of the draft inducer blower is the static pressure generated by the blower on the flue gases at a given air flow rate, wherein a blower is more efficient if it can generate higher pressures and air flows for a given power input to the electric motor which drives the blower impeller.
One known blower for a high efficiency furnace is shown in FIGS. 1–4, and generally includes a blower housing 20 having a housing body 22 and a housing cover 24. Housing body 22 is typically formed as a molded plastic component, having a cylindrical outer wall 26, a planar, annular top wall 28, and an axially recessed, planar, circular wall 30 to which electric motor 32 is mounted. Housing body 22 further includes an integral, tubular exhaust transition 34 projecting tangentially therefrom, having a circular outlet 36 to which an exhaust pipe (not shown) is connected. Housing cover 24 is a substantially flat, molded plastic circular plate which is attached to housing body 22 by being captured between housing body 22 and wall 38 of a furnace, as shown in FIG. 4. Specifically, a plurality of bolts 40 are inserted through respective mounting lugs 42 in housing body 22 and into a set of corresponding holes 44 in furnace wall 38 to thereby attach the blower housing 20 to the furnace. Holes 44 in furnace wall 38 are disposed in a standard pattern with a predetermined, fixed diameter, typically about 9.25 inches. An impeller 46, shown in FIGS. 2–4, is disposed within the interior of blower housing 20 between housing body 22 and housing cover 24, and is mounted for rotation upon drive shaft 48 (FIG. 4) of motor 32.
In operation, rotation of impeller 46 by motor 32 draws exhaust gases through a centrally disposed circular inlet 50 (FIG. 4) in housing cover 24 from the furnace into the blower housing 20, and the exhaust gases are discharged through outlet 36 of exhaust transition 34. Although the foregoing blower housing has proven to be effective for use with high efficiency furnaces, improvements to same are desired.
First, during the molding of housing body 22, tubular exhaust transition 34 is formed by a cylindrical-shaped exhaust transition mold (not shown). After the plastic material of housing body 22 cures, the exhaust transition mold is pulled outwardly from housing body 22 in a tangential or radial direction with respect to housing body 22. At least one other larger inner mold (not shown), which is cylindrically-shaped, is used to form the interior of housing body 22 and, after the plastic material of housing body 22 cures, is pulled away from housing body 22 along the axial direction with respect to housing body 22. Notably, it is not practical to shape the inner end of the exhaust transition mold to fit perfectly tangentially along the cylindrical outer surface of the housing body interior mold. Therefore, the exhaust transition mold is shaped to project radially outwardly from the cylindrical outer surface of the housing body interior mold a short distance. Thus, when housing body 22 is molded, the exhaust transition mold forms a recessed area 52 in exhaust transition 34, best shown in FIG. 3, which is radially offset from outer wall 26 of housing body 22. Problematically, this recessed area 52 defines an abrupt outward step or “bump” in the air flow through exhaust transition 34 which, as shown by the air flow arrows in FIG. 3, causes undesired turbulence and swirl in the air flow in recessed area 52 as the air flow passes through exhaust transition 34 toward outlet 36 of housing body 22.
Additionally, as may be seen from FIGS. 2 and 3, the intersection of the cylindrical exhaust transition mold and the cylindrical housing body interior mold which are used to form housing body 22 forms a sharp exhaust cutoff 54 within housing body 22, which is present in blower housing 20 and in many other known blower housings. Cutoff 54 is located proximate exhaust transition 34, and defines the point within blower housing 20 which separates the air flow through exhaust transition 34 from the remainder of the air flow within blower housing 20. As may be seen in FIGS. 2 and 3, the outer edge of impeller 46 is disposed very close to cutoff 54 to maximize the efficiency of air flow in blower housing 20 and to prevent back flow of air through the gap between impeller 46 and cutoff 54 into exhaust transition 34. As represented by the air flow arrows in FIG. 3, as impeller 44 rotates, a blade pass noise is generated as pressure waves exhausting the blade passages of impeller 46 impinge upon cutoff 54.
Known blower housings have included features for masking the foregoing blade pass noise. For example, a blower housing disclosed in U.S. Pat. No. 5,316,439 includes either a noise cancellation rod located within the outlet of the blower housing, or a nose-like projection projecting inwardly from the exhaust transition. Noise generated from one of the foregoing components interferes with, and substantially cancels out, the blade pass noise generated by the impeller blades passing the sharp cutoff. U.S. Pat. No. 5,484,259 to Ahmed et al. discloses a blower housing having a fin near the cutoff to provide a vortex in the air flow near the cutoff to reduce noise. However, these and similar methods only mask the blade pass noise, rather than eliminating such noise.
What is needed is a draft inducer blower housing for high efficiency furnaces which is an improvement over the foregoing.